Validated assessment tools for screen media use: A systematic review

Objective Accurate measurement of adult and child screen media use are needed to robustly assess its impact on health outcomes. Our objective was to systematically review screen media use measurement tools that have been validated against an objective “gold standard” tool. Methods The search strategy was initially conducted in Medline Ovid and translated to Embase, Web of Science, PsychInfo and Cochrane. A modified natural language search was conducted in Google Scholar and IEEE. The initial search was conducted in March 2021, and an updated search was conducted in June 2022. Additional studies were included from the references. Studies had to describe the validation of a tool to measure screen media use on participants of any age against a ‘gold standard’ or comparable objective measure. The COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) was used to assess the criterion validity. Four authors reviewed the titles in two rounds and extracted data. Results Twenty-nine articles were included in the review. Studies measured TV, computer, mobile device and social media site screen media use through: self or parent report, direct or video observation, computer and mobile device use tracking programs, and through other novel devices such as wearable devices and cameras. Correlations of self or parent report of screen media with the gold standard were lower than correlations of technology-based measures, and video observation with the gold standard. The COSMIN criterion validity ratings ranged from poor to excellent; most of the studies received a global score of fair or poor. Conclusions Technology based validated tools that more directly measure screen use are emerging that have been validated against a gold standard for measuring screen use. However, practical, objective measures of diverse types of screen media use that have been tested on diverse populations are needed to better understand the impact of screen media use on the development and physical and mental health of children and adults.


Methods
The search strategy was initially conducted in Medline Ovid and translated to Embase, Web of Science, PsychInfo and Cochrane. A modified natural language search was conducted in Google Scholar and IEEE. The initial search was conducted in March 2021, and an updated search was conducted in June 2022. Additional studies were included from the references. Studies had to describe the validation of a tool to measure screen media use on participants of any age against a 'gold standard' or comparable objective measure. The COnsensusbased Standards for the selection of health Measurement INstruments (COSMIN) was used to assess the criterion validity. Four authors reviewed the titles in two rounds and extracted data.

Results
Twenty-nine articles were included in the review. Studies measured TV, computer, mobile device and social media site screen media use through: self or parent report, direct or video observation, computer and mobile device use tracking programs, and through other novel devices such as wearable devices and cameras. Correlations of self or parent report of screen media with the gold standard were lower than correlations of technology-based measures, and video observation with the gold standard. The COSMIN criterion validity ratings ranged from poor to excellent; most of the studies received a global score of fair or poor. Introduction the bedroom; but underestimated TV time by over 3 hours/week when the child had a television in their bedroom [19]. When comparing parent report of young children's mobile device use with applications that track mobile device use, parents' estimates were inaccurate, with 34.8% over-estimating and 35.7% underestimating their child's device use [20]. To add to the confusion, many researchers have confounded sedentary time with screen media use by only using accelerometers to measure screen use behaviors [21], further limiting the accuracy of estimates of screen media use. Thus, the most common methods for assessing screen media use, self or proxy reports, are known to incorporate substantial error. A crucial step in identifying and assessing health outcomes from screen media use is accurately measuring when and for how long individuals engage in its use across multiple devices. This is especially important since screen media use may have a more adverse impact on health than overall sedentary time [22]. The validity of a screen media use tool can be assessed by comparing it to direct or recorded observations [23]. Other recent systematic reviews [16,[24][25][26] have reported on methods for measuring television viewing, mobile device usage and other screen media use, with a focus on self-report measures. The current systematic review evaluates screen media use tools that have been validated against a "gold standard" or previously validated tool. The quality of the available research is assessed to identify gaps in knowledge and inform future research on the validity of screen media use tools.

Methods
This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol for reporting systematic reviews [27]. (See S1 File for the PRISMA checklist) The protocol was registered in the PROSPERO International register of systematic reviews CRD42021240268 [28].
A medical librarian (AS) developed a search strategy in Medline Ovid (see S1 Table) and then translated the strategy to the Embase, Web of Science, PsycINFO, and Cochrane databases. An initial search was conducted in March, 2021, which included titles published on and before 2021. A second search was conducted, which included titles published between March 2021 and June 2022. Search terms included the following MeSH descriptors as well as synonymous key words and phrases: screen time, television, motion pictures, video games, computers, cell phones, smartphones, internet, social media, computer-assisted instruction, time factors, behavior observation techniques, surveys and questionnaires, validation studies, and reproducibility of results. Additional modified natural language searching was conducted in Google Scholar and IEEE Xplore. The initial search produced 24,058 results, and the second search produced 4,183. Duplicates were removed after each search.
To be included in the review, studies must have described the validation of a tool to measure screen media use against a gold standard. Studies could include participants of any age, living in any part of the world. Studies had to be reported in the English language. No publication date limits were imposed. Reports from the gray literature (such as government reports, proceedings, dissertations, and theses) were not specifically searched. Articles were excluded if they did not report on the validation of a tool to measure screen media use. Studies focusing on phone calls and text message mobile device use only, but not mobile device screen time, were also excluded. The references of all included articles were further searched, and 6 additional articles were included from this source that met the inclusion/exclusion criteria and had not been found in the initial search.
Seven authors (AS, TD, AB, OP, SC, TG, OH), screened the titles and eliminated publications which clearly did not meet inclusion criteria. Each title was reviewed by at least two authors independently. Secondly, the remaining articles' abstract and/or full text were reviewed by two authors independently to identify those which met the inclusion and exclusion criteria. Discrepancies about which articles to include or exclude were discussed with the entire research team until a consensus was reached.
Twenty-nine articles emerged which fit the inclusion and exclusion criteria. The data from the articles were extracted independently by authors working in pairs (TG, OH, AB, OP, SC) and results were compared by a third author for accuracy. Once the data abstraction was completed and compared, discrepancies were discussed by the research team until a consensus was reached. Data and data extraction forms were stored in a custom-made Access database.
Data extracted from each study included basic information such as sample size, year published, country where study was conducted, and sample characteristics. For study methods, the team extracted the media measured (television, desktop, laptop, gaming system, mobile phone, tablet, movie screens, e-readers, or other), measurement tool type being tested 1) selfreported or proxy-reputed assessment tool (media diary, recall interview, questionnaire, etc.), or 2) technology-based assessment tool (computer software, mobile device app, wearable cameras of devices, etc.); description of the objective measure used for comparison (direct observation, video observation, or other), setting (lab, in-home, public, or other), statistical metrics conducted, and a description of the research question. Extracted information about each study included age group (adults, children, unknown), and additional characteristics within age groups (preschool children, students, office workers). Race and ethnicity were extracted when available. In the case of studies which included multiple objectives or subsamples, data were extracted only from the population in which screen media use was measured and validated.
The methodological quality of studies was assessed using the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) checklist specific for assessing criterion validity of studies [29]. COSMIN was designed to calculate a quality score for measurement properties. The items assessed by COSMIN were: 1) are missing items reported, 2) is there a description of how missing items were handled, 3) is the sample size adequate, 4) is the criterion a reasonable 'gold standard', 5) are there important flaws in the study, 6) do they report correlations or Area Under the Receive Operating Characteristic (ROC) Curve (AUC), and/or 7) is sensitivity and specificity determined. Four authors (AB, OP, TG, and OH) independently assigned a COSMIN rating of poor to excellent for each item per study. The agreement between coders was substantial [30][31][32] with a weighted kappa for all coders combined of 0.66 (95% CI 0.57-0.75). After kappas were calculated, the four research staff discussed discrepancies and agreed upon a final COSMIN rating for each item. According to a COSMIN scoring convention, the quality score per measurement property was obtained by taking the lowest rating of any item, or 'worst score counts' [29].

Results
Following initial screening of 28,257 article titles, 26,452 were eliminated because they did not meet the inclusion criteria. The resulting 1,805 articles were assessed for eligibility by reviewing the abstract or full manuscript. Twenty-three articles met the criteria. Six additional articles emerged from the reference lists of relevant studies, and were also included, resulting in 29 articles that met inclusion criteria and which were included in the review (Fig 1). A description of the study participants of the 29 studies are summarized in Tables 1 and 2 describes the media measured, setting, measurement tools, and relevant statistical metrics of the included studies, organized by self-report or technology-based assessment tools. Sample sizes ranged from 2 to 1,211. The majority of studies were conducted in-Home (n = 9), followed by the workplace (n = 8), no location reported (n = 3), in a lab (n = 2), or in a combination of these locations (n = 7).

Correspondence between screen media use measurement and a gold standard
Television screens. Two studies compared a self-report with direct or video observation of TV viewing during the same time frame [23,56]. Anderson et al. [23] found a correlation of 0.84 between the TV diary completed by parents and observation of child TV viewing using video recordings, and a correlation of 0.27 between a global estimate of TV viewing completed by the parent and video observation. Bechtel et al. [56] reported a 43.6% agreement between TV viewing questionnaire and staff coded video observations of TV viewing. Three studies compared self-report measures to a TV monitor such as TV Allowance™ (Family Safe Media, Park City, UT) as an objective measure of TV screen media use [19,40,51]. TV Allowance™ was a device which connected to a television or computer power cord and detected when the Mobile phone In-Home Self-report questionnaire Phone use tracking app (custom made) Yes 1-week Bland-Altman plots (difference between self-report and tracking app lies within 2 SD from the mean and is more variable with increasing minutes of use); Self report of typical phone use is 1.75 to 3.16 times higher than phone tracking app.
(Continued )   device was turned on (but not if someone was watching). It did not work on all TVs and is no longer available for purchase [60]. Robinson et al. [19] reported a correlation of 0.49 between parental report of child's weekly TV viewing and the TV monitor. Otten et al. [40] found a correlation of 0.54 between a one item adult report of screen media use and the TV monitor installed in the participant's homes. Mendoza et al. [51] reported correlations between 0.45-0.55 between child's TV viewing duration as reported in a TV diary by parents versus the duration reported by the electronic TV monitor and correlations of 0.47-0.51 between the TV diary and an Ecological Momentary Assessment (EMA) approach, which prompted participants to answer brief surveys throughout the day about the child's current behavior (i.e. "what activity is your child doing now?"). In addition, Mendoza et al. [51] used accelerometers to measure the child's sedentary time and found correlations of -0.04-0.18 between the TV diary and sedentary time.
Only one study tested a technology-based assessment for measuring TV viewing compared to a gold standard. Vadathya et al. [52] used an automated device (FLASH-TV) to measure TV viewing of a target child. The device detected the presence of the target child in the room, differentiated their face from other persons in the room, and detected when the child was gazing at the TV using machine learning processing of video data captured in front of the TV. They found an Intraclass correlation (ICC) of 0.725 between the FLASH-TV estimation of the target child's TV viewing duration and staff coding of the video records.
Computer screens. Four studies tested self-reported assessments of computer use to a gold standard, including direct observations, a computer use monitoring software, WorkPace (Niche Software Ltd., New Zealand), and internet use tracking software. Three studies exclusively measured self-report questionnaires against computer monitoring software [36,46,47] and one [42] compared self-report with video observations of participants' computer use. Studies which used self-report measures had weak correlations to the gold standards [61]. Faucett & Rempel [42] reported a correlation of 0.50 between self-report and direct observation of computer use. Mikkelsen et al. [46] found a correlation of 0.51 of their self-report questionnaire and the computer use monitoring software. IJmker et al. [47] reported an 18% agreement between self-report and computer use monitoring software; Araujo et al. [36] reported correlations of 0.29 (recall of computer use the day before) and 0.29 (recall of computer use on an average day) with their custom-made internet use tracking software which captured each URL (Uniform Resource Locator, a web page address) accessed by the participant, as well as time spent on each URL.
An approach tested by Homan & Armstrong [38] was a custom-made electronic activity monitoring device consisting of external microprocessors which sensed keyboard, mouse and computer use. Two studies compared the computer monitoring software [44,45] with video observations of participants' computer use with correlations ranging from r = 0.87-0.92, and 0.93-0.94, respectively.
Three studies [38,41,43] used a triangulated approach by comparing self-report, technology-based and direct observation measurements. The technology-based programs, WorkPace and KBlog, work similarly to the Homan & Armstrong approach [38] by recording dynamic mouse and keyboard use and estimated total computer use based on these indicators. These three studies found stronger correlations between the direct observation and the technologybased measures, than the self-report measures. Homan & Armstrong [38] found a correlation of 0.78 with self-report and 0.93 between their custom-made electronic activity monitoring and video observations of computer use. Similarly, Douwes et al. [43] found a correlation of 0.41 between self-report and direct observation of computer use, in contrast with a correlation of 0.86 between their computer use monitoring software and direct observation of computer use. Yeh et al. [41] found a correlation of 0.387 between self-report and video observations of computer use, and a correlation of 0.960 between the computer monitoring software and video observation of computer use for the same period.
Mobile device screens. Seven studies compared self-or proxy-report of mobile device use (smart phone or tablet) to a device-use tracking application (app), including custom-made [33,48,59] and commercially available tracking apps [20,33,48,50,53,54,59] such as Chronicle [20,53], SDU DeviceTracker [50], and Effortless Assessment of Risk States (EARS) [54]. Phone tracking apps are similar, in that they collect usage data from the operating system of Android mobile devices (phones and tablets) and record the name of the app used and duration of usage. Researchers can collect a report of the apps used on the device for the duration of the study. Barr et al. [53] compared parent-reported media diaries and reports of child's mobile device use with the Chronicle app and found Kendall's tau-b rank correlations ranging from 0.41-0.81. Berolo et al. [33] presented Bland-Altman plots showing that self-report of mobile screen media use was 1.75 to 3.16 times higher than their tracking app. Radesky et al. [20], reported 25.7% agreement between parent report and the Chronicle tracking app. Lee et al. [59] found a weak correlation of -0.1 between self-report of overall mobile screen use and the tracking app (See Table 2 for additional correlations found for categories of phone use). Geyer reported a correlation of 0.19 between their custom-made mobile use tracking app and self-report [48]. Wade et al. [54] reported correlations ranging from .1-.49 between the EARS app and self, or parent report of mobile device use.
Apple mobile devices, which function with the iOS operating system, have restrictions that currently prevent similar usage tracking apps to measure device use. To circumvent the restrictions, Ohme et al. [39] and Radesky et al. [20] asked participants with Apple devices to provide periodic screenshots of their Apple Screen Time application, which provides a report of the device's usage. In addition to screenshots of iOS devices, when comparing parents' weekly estimates of their child's screen media use, Radesky et al. [20] found 31.3% agreement between the parent report and the iOS screenshots. Ohme et al. [39] exclusively compared mobile screen media use self-report questionnaire (on adults) and iOS screen time screenshots and found a correlation of 0.3711 between the two.
Two studies assessed technology-based approaches for measuring mobile device use to a gold standard. Kristensen et al. [50], designed a custom-made tracking app (SDU Device-Tracker) which is able to collect mobile use tracking data on iOS and Android devices. They found a correlation of 0.99 between their tracking app and Android's built-in ActionDash application. In contrast, they found a lower correlation of 0.88 between their tracking app and Apple Screen Time application. Trabulsi et al. [49] compared the performance of eye tracking glasses, Tobii Pro Glasses, versus human coded video data of the participants' eye movements using specific eye-tracking filters, or algorithms. The highest correlations they found were 0.972-0.975, between their NoMerge_NoDiscard (see Table 3 for specific distinctions between the filters used) filter and human coded video data.

Social media use.
Three studies focused on screen media use spent on specific social media [34,37,55]. Junco [34] utilized a computer use monitoring software to compare selfreported time spent on Facebook, Twitter, as well as time spent on email and searches. They found correlations of 0.587 between self-report of time spent on Facebook, and 0.866 time spent on Twitter against the actual usage measured by the software (see Table 2 for additional correlations of other types of computer use). Henderson et al. [37] utilized a Python script to access and download participants' activity on Twitter and compare it to their self-reported usage. They found correlations of 0.00 to 0.24 between self-report of Twitter use and downloaded data. Verbeij et al. [55] measured adolescents' time spent on various social media platforms by comparing self-report questionnaires, the Ethica mobile use tracking app, and Experience Sampling Methodology (ESM) which was comprised of periodic text messages to  [55] found a correlation of 0.55 between ESM and the mobile tracking app; 0.59 between self-report of total social media use on a typical week and the mobile tracking app; and 0.65 between self-report of total social media use on the previous week and the mobile tracking app.
Tools for measuring multiple screen media platforms. Fletcher et al. [58] and Zhang & Rehg [35] used novel devices to measure screen media use on a variety of screens. The former tested an optical color sensor with accelerometer worn on the wrist. The sensor was tested while the participants performed various activities in front of a TV, computer or laptop screen, and under varied lighting conditions. Fletcher et al.'s [58] preliminary data suggest their machine learning algorithms can differentiate the type of screen to which the participant was exposed (TV, computer or laptop). They reported scores of AUC of 0.90 for detecting a TV screen, 0.89 for detecting a computer screen, 0.83 for detecting both screens are present near the device. Zhang & Regh [35] tested a head-mounted wearable camera compared to eye tracking glasses, TV detector and video observation. They used machine learning algorithms to analyze and classify the video recorded by the wearable camera to identify when the participant was watching TV, or a variety of screens. Zhang & Regh [35] reported precision (percentage of image frames correctly classified by their system as the participant was watching the screen compared to video observation of TV viewing) of 0.917, and recall (percentage of image frames correctly detected as participant watching the screen compared to video observation) of 0.945 of the head-mounted wearable camera in identifying the TV screen. Additionally, they reported an AUC of 0.98 between Precision and Recall detecting multiple screen viewing (see Table 3 for additional sub-study findings).
Content of screen media measured. The tools described by Berolo et al. [33], Radesky et al. [20], Geyer et al. [48], Lee et al. [59], Wade et al. [54], Junco [34], Henderson et al. [37], Verbeij et al. [55], and Trabulsi et al. [49] captured the content of the screen media being measured; for example, if the participant viewed entertainment, educational, or other content. Barr et al. [53], Ohme et al. [39] and Zhang & Rehg [35] reported the screen media tool they used was capable of measuring content, but the published study did not include that analysis. None of the other studies described in this review included a measure of the content of screen media use. Table 3 reports the methodologic quality of the included studies. Four studies were rated as Excellent [23,46,47,49], four as Good [38,39,43,54], twelve as Fair [19, 34, 36, 37, 40, 48, 50- [20,33,35,41,42,44,45,56,58]. The poor ratings were due to a small sample size (<30) [35,41,42,44,45,56,58] and/or not reporting correlations or AUC in their results [20,33,56]. As per COSMIN, item 5 (flaws in design or methods) articles received a rating of fair if they had minor methodological flaws in the design or execution of the study such as having a small sample size, not reporting correlations or AUC, or they described a flaw in the design of the questionnaires they used [36,37].

Discussion
This review identified and described tools that have been validated against gold standard assessments to measure screen media use. Our review is unique and builds upon existing literature by identifying novel technology-based methods that measure screen media use. Such approaches may be more objective than self-report approaches but are still relatively early in their testing and use. Technology-based measures of screen media use, including TV monitors; internet, computer and mobile use tracking software; wearable cameras; wearable devices, and image processing machine-learning approaches, had higher correspondence with screen media use detected by direct and video observation (ranging from 0.73 [52] to 0.99 [50]) than self-report questionnaires (ranging from 0.00 [37] to 0.84 [23]). Self-or proxy-report via TV and media diaries [23] tended to have better correspondence to gold standard (0.84 [23]) than short survey style estimates of screen media use (0.54 [40], 0.50 [42], 0.51 [46], 18% agreement [47], 0.291-0.294 [36]). This is consistent with the systematic review by Parry et al [26], which found studies comparing self-report such as questionnaires and global estimates to objective measures tended to report low correlations; except for TV diaries, which reported moderate correlations with video observations and electronic TV monitors. The current review adds to this by finding correlations between technology-based approaches for measuring screen use and direct or video observations were high to very high. A previous systematic review by Byrne et al. [16], noted the methods for assessing television viewing time had not kept pace with current research interest. They were not able to find any objective, device-based method of assessing screen media use. The present review identified a number of emerging objective methods to measure screen media use on a diversity of screens. Three studies in this review (Zhang & Regh [35], Fletcher et al. [58] and Vadathya et al. [52]) relied on technology-based approaches to measuring screen media use in combination with machine learning technology. These innovative approaches could transform research methods for measuring screen use, however, the samples were small and the methods warrant further testing. Computer and internet use tracking software programs such as the ones used by Homan & Armstrong [38], and Mikkelsen et al. [46] used motion trackers to measure hand and arm movements while using the keyboard and mouse, summing them and equating these to total computer usage time. These studies came from the fields of ergonomics and occupational health, wherein the researchers were not interested in measuring screen media use, but in finding objective measures of overall computer use to establish a relationship with postural problems and musculoskeletal pain. Other studies relied on existing technologies such as electronic TV monitors which detected whether the TV was on, but not whether an individual was actually gazing at the TV [19,51]. One study assessed the use of accelerometers to measure sedentary time, as a proxy for screen use [51]. Another study reviewed the use of accelerometry or heart rate monitoring as a proxy to measure of TV viewing time [62]. Both posed additional challenges as not all screen media use is sedentary (e.g. active video games) and not all sedentary behavior involves use of a screen (e.g. reading a book, working on a puzzle, sitting and talking with friends). Clear guidelines to separate screen use and sedentary behaviors during assessments are available from the Sedentary Behavioral Research Network [21].
Most of the studies in this review measured television, computer, or mobile devices separately, limiting the ability to assess a participant's total screen media exposure across different platforms. While TV viewing is still common, particularly among children [8], screen media use behaviors are evolving. Adults are spending less time on traditional televisions and spending more time on mobile devices, viewing online television, and gaming on consoles [2]. The significant rise in mobile devices has created a new ubiquitous medium where individuals spend time viewing screens. Tracking apps which capture mobile device use have been used as an objective measure of mobile screen media use because they do not rely on participant recall. Rather, they 'read' the device's usage log. However, no studies compared mobile device tracking apps with video or direct observations. Video or direct observation is the current gold standard to measure screen media use; but it is time intensive, invasive, and in the case of mobile devices, impractical. However, such tracking apps are limited in measuring a person's device use when the device is shared by multiple people. Assessment of mobile device use among young children can be more challenging because they are more likely to share a device with a sibling or parents [20]. Nevertheless, this technology has opened a new area of study of when and for how long individuals, especially adolescents and adults, engage with their mobile devices. Despite this, self-report continues to be widely used to measure screen media use [16,24], even though the validity is low.
Few studies reported on the race and ethnicity of their samples. Those that did, tended to include White adults from the United States and Europe. Including participants from diverse backgrounds, ages and physical appearance is relevant because some screen media use detection devices, (such as the one used by Zhang & Rehg [35] and Vadathya A K., et al., [52]) rely on image processing of the participant's face and/or eyes using machine learning algorithms. Skin color and lighting can influence the ability to detect screen media gaze. Diversity of participants in training and testing such approaches reduces the likelihood that these measurement methods will contain inherent bias [63]. Some are considering this in their technology development pipeline [52]. Similarly, the wrist-worn detector used by Fletcher et al. [58] is affected by movement, and children may be more likely to be physically active during screen use than adults. Additionally, screen media use measurement tools that work on adults may not work on children due to different types of screen use or recognition patterns. Validation studies are needed among children as well as adults.
Most of the studies in this systematic review were scored as poor or fair, using the COSMIN checklist for criterion validity. The low ratings were mostly due to methodological flaws, or small, non-representative numbers of participants.

Limitations
The search strategy may have missed relevant articles due to the large number of studies interested in measuring screen media use. For example, the search strategies captured public health, psychology, medical and some educational journals. The search also included a natural search in the IEEE engineering database to capture new technologies from the engineering side. However, other fields of study, such as communications, may not have adequately been covered, making it possible that we may have missed relevant articles. Additionally, the key words used may have missed some articles in the ever-expanding field of screen media use. The review was limited to English-language articles and those published in peer-reviewed journals; the gray literature was not searched, all of which limit the scope of the review.

Conclusions
Practical self-report measures of screen media use tend to be inaccurate when compared to gold standard assessments of screen media use. Technology-based assessment tools for measuring screen use, such as tracking apps, cameras, light sensors and image processing machine learning algorithms, demonstrated much higher correlations to gold standard assessments, but many are still in developmental stages and need further validation before they can be deployed. Studies are needed on the development and validation of accurate, but simple to deploy, technology-based measures of screen media use, especially with diverse populations including children and racially and ethnically diverse samples. This may facilitate understanding the impact of screen media use on academic performance, physical and mental health, and development among children and adults from a variety of backgrounds, and may demonstrate different impacts from different screen media exposure. These studies need to integrate data from diverse screen platforms and account for the fluid nature of multiple screen media use by people, the content of what is viewed, and multi-tasking across screens.